This invention relates to apparatus for transferring material from one hopper to another and, more particularly, for transferring relatively pure abrasive material under conditions preventing contamination of the material and preventing the material from damaging various parts of the apparatus.
In processing abrasive, granular or powder materials, it is often necessary to transfer the material between work stations. Certain of these materials are easily contaminated and highly abrasive and these phenomena impose special requirements on the transfer apparatus. As an example, aluminum oxide (Al.sub.2 O.sub.3) in granular or powder form is used in growing saphhire crystal for use as a substrate for various integrated circuits. Because of the stringent purity requirements of the electronics industry, this raw material is normally "outgassed" in a vacuum-tight hopper to remove oxygen, water, and other contaminants that may be contained therein. After outgassing, the now relatively pure material is transferred to a vacuum-tight feeding apparatus for feeding it to a crystal growing apparatus. Thus, it is a special requirement that the transfer apparatus between the hopper and the feeding apparatus be air-tight to prevent recontamination of the raw material.
This special requirement is complicated by the fact that the outgassing of aluminum oxide can take from 6 to 12 or more hours. It is thus desirable to outgas the aluminum oxide at a work station separated from the feeding apparatus so that the feeding apparatus and the crystal growing apparatus can operate while the material is outgassed. In addition, it is desirable that the outgassed material be transferable to the feeding apparatus without shutting down this apparatus. Thus, added to the special requirement above are the additional special requirements that the vacuum-tight hopper be easily connected to and disconnected from the feeding apparatus and that contamination of the raw material in the hopper and the feeding apparatus be prevented when the hopper is disconnected and when it is reconnected.
In addition, aluminum oxide is a highly abrasive material capable of destroying certain of the parts normally used in the construction of transfer apparatus. For example, the transfer apparatus may include a valve for controlling the flow of material from the hopper to the feeding apparatus. Such a valve would include a valve member, a valve seat and appropriate seals, all or any of which can be scored or marred on contact with even the smallest particle of aluminum oxide. If this scoring or marring occurs in the area of the valve sealing surfaces, the sealing integrity of the valve is lost. Thus, it is an additional special requirement that the transfer apparatus be arranged to prevent the material from impinging on sealing portions of the valve.
It should be understood that these special requirements are in addition to the usual requirements that the transfer apparatus be economical, rugged, reliable, relatively maintenance free, and easy to use.